In MS-DOS (trade name) popuralized as an operating system for personal computer in recent years, both directory management information and file data are recorded together in the same recording units in the same area. This method advantageously constructs a hierarchical directory file system without making distinction between a method for management of file and a method for management of directory.
In the method of UNIX (trade name), the recording area is divided into Data Block and Super Block. Directory file, sub-directory file and data file are recorded in the Data Block, and i-node list is recorded in the Super Block.
In the directory file, name of file belonging to a corresponding directory and i-node No. are described. The i-node list consists of a plurality of i-nodes to which predetermined numbers are attached, and respective i-nodes describe recording positions (locations) within Data Block of the sub-directory file and the data file.
Further, in the method of Macintosh (trade name) disclosed, e.g., in the U.S. Pat. No. 4,945,475 and the Japanese Patent Application Laid Open No. 16132/1988, etc., directory management information are caused to all undergo hierarchical management by making use of the structure of B-Tree in Catalog File. Namely, collective management of all of files and directories is carried out by keys of records of B-Tree (combination of ID (Identification) Nos. of parent directories and file names. This catalog file is recorded in practice in a concentrated area. For retrieval of data file, such B-Tree is used in place of directly employing hierarchical information of directory so that any file in the hierarchical directory is indirectly retrieved.
However, in the hierarchical directory file system of MS-DOS, since information of respective directories are recorded in a distributed (dispersed) manner onto a recording medium, there is the possibility that files existing at lower rank layers of the hierarchical directory must undergo seeking (search) over the entirety of the recording medium for retrieval of directory. As a result, in the case of recording media where access speed is low, remarkably lowering of file access speed would result.
In the case where directory management information are recorded in a distributed manner as described above, the number of physical write/read operations for updating of directory is increased, and buffer utilization efficiency of the directory management information also becomes poor.
Moreover, in the case of the method of UNIX, while the number of physical write operations of Super Block can be reduced by carrying out buffering, since management of i-node itself is carried out by a method independent of the directory hierarchy (i-node list), a method for management of these i-nodes becomes complicated. Accordingly, this management method is not effective for compact (small-sized) equipments such as personal computer, etc. aside from higher performance work station.
Further, since the management by B-Tree is extremely complicated and an access to sub-directory is indirectly provided in place of directly making an access to sub-directory by using directory hierarchical information, it is necessary to retrieve node trees of B-Tree every retrieval operations of directory.
Since B-Tree is of a structure such that distances from the root of the tree up to all leaves are the same (the same number of stages), retrieval is efficiently carried out. However, according as the numbers of directories and/or files is increased, such distances (the number of stages) are increased in a manner of three stages, or four stages. As a result, the number of access operations has a tendency to substantially increase as compared to the case where location (position) of directory is directly indicated. For this reason, this method is not efficient and is not suitable for compact (small-sized) equipments.
This invention has been made in view of such circumstances, and contemplates permitting retrieval of a predetermined directory file in the hierarchical directory by a simple method, and providing realization of high speed file retrieval and reduction of the number of physical read/write operations for updating of directory. In addition, this invention contemplates permitting directory information to be updated in a short time.